The present invention relates to tape storage systems, and more specifically, to linearity/nonlinearity of servo patterns.
Timing-based servo (TBS) is a technology which was developed for linear tape drives in the late 1990s. In TBS systems, recorded servo patterns include transitions with two different azimuthal slopes, thereby forming a chevron-type pattern. These patterned transitions allow for an estimate of the head lateral position to be determined by evaluating the relative timing of pulses generated by a servo reader reading the patterns as they are passed over the servo reader.
In a TBS format, the servo pattern is prerecorded in several bands distributed across the tape. Typically, five or nine servo pattern bands are included on a given tape which runs about parallel to a longitudinal axis of the tape. Data is recorded in the regions of tape located between pairs of the servo bands. In read/write heads of linear tape-open (LTO) and IBM Enterprise tape drives, two servo readers are normally available per head module, from which longitudinal position (LPOS) information as well as a position error signal (PES) may be derived. Effective detection of the TBS patterns is achieved by a synchronous servo channel employing a matched-filter interpolator/correlator, which ensures desirable filtering of the servo reader signal.
With the increase in track density that is envisioned for future tape media and tape drives, accurately controlling the lateral position of the head and/or skew of the head with respect to tape by using feedback generated by reading the TBS patterns becomes increasingly difficult. Conventional servo based implementations may not be sufficiently accurate to ensure adequate positioning of the data readers and writers that move along data tracks. Furthermore, the repetition rate of the head lateral position estimates may be too low to ensure proper track-following operation as tape velocity varies during use. The repetition rate of the head lateral position estimates may additionally be unable to support future actuators with larger bandwidths.